The fact that human mothers have support from family while they’re breast-feeding may be a key strategy that enables humans to reproduce more rapidly than other primates, new research suggests.
Social support helps mothers conserve energy in a way that allows their bodies to prepare for their next pregnancy.
“Humans out-produce other primates. So we are examining to what degree this is related to our cultural flexibility,” said Barbara Piperata, assistant professor of anthropology at Ohio State University and principal investigator of the research.
Piperata’s research in the Brazilian Amazon suggests that social support can make a substantial difference in how much energy new mothers can conserve while they are breast-feeding.
Piperata described the research February 13 during the annual meeting of the American Association for the Advancement of Science in Chicago. She has collaborated on the work with Debbie Guatelli-Steinberg, associate professor of anthropology at Ohio State.
Breast milk production places huge energy demands on women’s bodies – an estimated 30 percent increase. But humans have multiple ways to offset those demands that involve more than just eating more or doing less.
Some studies have suggested the human body becomes more metabolically efficient during lactation, requiring less energy or less oxygen to complete physical tasks. And new human mothers also tend to have other humans around to share the work burden.
Nonhuman primates, which have similar energy demands while breast-feeding single, slow-growing offspring, don’t have that same flexibility. As a result, their reproductive rates are relatively low, averaging a new birth every four to seven years.
“We know that negative energy balance on the body lowers a female’s ability to get pregnant. If humans mediate that, have social support, and are able to maintain or even achieve a positive energy balance, they can get pregnant faster. From an evolutionary perspective and fitness, that’s important,” Piperata said.
Primates differ from many other mammals, especially those that give birth to litters of multiple offspring that grow rapidly. These species, such as rodents, produce energy-dense milk that saps much more of their energy, and they typically eat up to 200 percent or 300 percent more calories per day to make up for that loss.
Producing dilute milk for a single offspring requires a smaller increase in energy, giving nonhuman and human primates more options to meet that energy demand. But the similarities might end there, Piperata said.
“We really do have a primate pattern. We are just a different great ape. Our biology reflects it,” she said. “I think we’re using our culture to really expand in many, many ways, and reproductively, social support has been an important avenue.”
In the Brazilian Amazon where Piperata has been working, the average length of time between births is 27 months. Piperata’s examination of this society’s postpartum practices has enabled her to document how women conserve energy immediately after childbirth and throughout the lactation period, which averages about 18 months.
In this Ribeirinha culture, women observe resguardo, a period of 40 or 41 days after birth during which they are restricted from performing strenuous household tasks. Piperata found, paradoxically, that despite their low levels of physical activity, these lactating women were unable to consume sufficient calories to meet their energetic needs because they were restricted from performing the very household tasks that involved food production and preparation.
Even though resguardo does not necessarily improve maternal nutrition for every new mother, the practice persists. In a paper published in a recent issue of the journal Social Science and Medicine, Piperata concluded that the society continues to observe resguardo because it provides an important social role in women’s lives and allows new mothers to spend more time on infant care.
She has new data suggesting that social support affects how much energy the new mothers in the Amazonian culture are able to conserve during lactation.
“Most of their diet comes from their own investment in getting it. There is no electricity, no running water, no refrigeration. If they expend energy to get food, then there is a problem,” she said. “Women who had social support during lactation were able to avoid subsistence work for a long period of time, lowering their energy expenditure. They also had help meeting their dietary needs, especially during the early period of lactation. The women who did not have social support had a more difficult time meeting their caloric needs and suffered a great deal.”
And that scenario, even in conditions without technological machinery to get work done, demonstrates the stark difference between the human and nonhuman primate lactation experience.
“What is it that we can do that other primates don’t do? It’s social support. We don’t see that in great apes at all. Females are pretty much on their own. Nobody feeds them or their offspring during lactation,” Piperata said.
Piperata plans to extend this work to explore the role of other household members, including grandmothers and older children – in helping new mothers conserve energy during lactation. Among Ribeirinhos, new mothers are often supported during lactation by their own mothers and mothers-in-law, as well as their own children.
Rather than just documenting what the women do to provide social support, Piperata plans to quantify exactly how that assistance offsets new mothers’ energy costs during lactation and to determine whether that support might reduce the amount of time between births as a result.
This work is supported by the Wenner Gren Foundation for Anthropological Research Inc. and the National Science Foundation.
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